The present disclosure relates to controllers for power converters such as power factor correction circuits.
Light Emitting Diodes (LEDs) can benefit from being driven with a constant average current. Power regulators that provide power to LED circuits include both a comparator and a gate driver that have a delay. An inductor current will overshoot past an upper threshold, and undershoot below a lower threshold. Even if a turn-on delay is the same as a turn off delay, the rate of change of the inductor-current when it is rising is often not the same as when it is falling. This can result in an undesirable shift of the average current because the magnitude of the overshoot is not the same as the magnitude of the undershoot. That is, the delays in the comparators and a gate driver may lead to an average current that deviates from the target value. This can be caused by the delay in the comparator hysteresis and gate driver, and causes overshoot and undershoot which depends on the change in inductance over time (dl/dt). Furthermore, the size of the deviation depends on the actual output voltage.
Such a deviation can be a particular problem for so called matrix beam applications. For matrix beam applications, all LEDs are connected in series with each other, and may need to be dimmable individually. This can be done by selectively shorting some LEDs in the string with a PWM (pulse width modulated) signal. A problem can arise because shorting the LEDs causes the total string voltage to change, which as discussed above can result in an undesirable shift in the average output current.